Reduce du/dv shifts in Bump() when no differentials are available.
Reduce du/dv shifts in Bump() when no differentials are available.

Light paths in BDPT don’t have ray differentials; in turn, when computing
bump map surface normals, we would filter over too large an area,
which in turn gave results that were quite different than were
computed for camera paths. When visible points on the light path
were then splatted into the film, we’d see artifacts.

Significantly reducing the ad-hoc filter region used for light paths
helps a lot, though it would be nice to have a more robust solution
here..

Issue #87.

nyue/pbrt-v3,shihchinw/pbrt-v3,shihchinw/pbrt-v3,nispaur/pbrt-v3,mmp/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,nispaur/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,nispaur/pbrt-v3